1. Field of the Invention
The present invention generally relates to piezoelectric thin-film filters and filters using the same.
2. Description of the Related Art
Recently, there has been an increasing demand for filters with compact and light resonators and filters configured by combining these filters as wireless communication equipment that may be typically cellular phones has spread rapidly. In the past, dielectric filters or surface acoustic wave (SAW) filters were mainly used. Nowadays, piezoelectric thin film resonators and filters using these resonators are attracting attention, in which the piezoelectric thin film resonators are characterized in that they gave good high-frequency performance and are compact and producible as a monolithic device.
The piezoelectric thin-film resonators may be categorized into an FBAR (Film Bulk Acoustic Resonator) type and an SMR (Solidly Mounted Resonator) type. Japanese Patent Application Publication No. 60-189307 and Japanese Patent Application Publication No. 2004-200843 disclose piezoelectric thin-film resonators of FBAR type. FIG. 1 show a conventional FBAR type resonator and a conventional SMR type resonator in parts (a) and (b), respectively. A lower electrode 12, a piezoelectric film 14 and an upper electrode 16 are laminated in this order on a substrate 10 having a cavity 18 or an acoustic multilayer film. The cavity 18 or acoustic multilayer film is formed below an overlapping portion in which the upper electrode 16 and the lower electrode 12 overlap each other across the piezoelectric film 14. Such a portion is referred to as a resonance portion 22. The cavity 18 in the FBAR may be formed by dry or wet etching from the backside of the substrate 10, which may be made of silicon. The cavity 18 may be formed so as to be positioned between the lower electrode 12 and the substrate 10 by removing a sacrificed layer provided on the surface of the silicon substrate 10 by wet etching. In this case, the cavity 18 may be called a gap. The acoustic multilayer film in the SMR has first layers having a relatively high acoustic impedance and second layers having a relatively low acoustic impedance so as to be laminated alternately to a thickness equal to λ/4 where λ is the wavelength of an acoustic wave of the resonator.
The upper and lower electrodes 12 and 16 may be made of aluminum (Al), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), ruthenium (Ru), rhodium (Rh), iridium (Ir) or the like. The piezoelectric thin film 14 may be made of aluminum nitride (AlN), zinc oxide (ZnO), lead zirconium titanate (PZT), lead titanate (PbTiO3) or the like. The substrate 10 may be made of glass other than silicon.
The piezoelectric thin-film resonator of FBAR or BAW type has a loss caused by leak acoustic waves 30 propagated outwards from the resonance portion 22, as shown in parts (a) and (b) of FIG. 1. Portions that are located further out than the resonance portion 22 are referred to as non-resonance portions. The leak acoustic waves 30 propagated through the non-resonance portions are not converted into electric signals and cause a loss. This phenomenon is called lateral leakage of the acoustic wave 30. One of factors that cause the lateral leakage of the acoustic wave 30 is the difference in the acoustic velocity between the resonance portion 22 and the non-resonance portions. In other words, there is a factor that depends on the Poisson's ratio of the piezoelectric film 14. When the Poisson's ratio of the piezoelectric film 14 is equal to or greater than 1/3, the acoustic velocity in the resonance portion 22 is less than the acoustic velocities in the non-resonance portions. When the Poisson's ratio is less than 1/3, the acoustic velocity in the resonance portion 22 is greater than the acoustic velocities in the non-resonance portions. For the piezoelectric film 14 having a Poisson's ratio of 1/3 or greater, an appropriate mass is applied to the resonance portion 22, so that the acoustic velocity in the resonance portion 22 can be made less than the acoustic velocities in the non-resonance (peripheral) portions and the lateral leakage of the acoustic wave 30 be restrained easily.
However, when the piezoelectric film 14 has a Poisson's ratio of 1/3 or lower, the relation in the acoustic wave in the absence of the lateral leakage is opposite to the above-mentioned relation. Thus, the lateral leakage of acoustic waves cannot be restrained easily. For example, aluminum nitride used for the piezoelectric film 14 has a Poisson's ratio equal to or less than 1/3. Thus, it is difficult to restrain the lateral leakage and thus reduce the loss.